Portable electrical devices have gained widespread market acceptance, and have taken numerous forms. Cellular telephones, portable computers, and cordless power tools are but a few examples of such devices. For any such device to be successfully marketed, attention must be given to the means of powering the device. Consideration must be given to the reliability, cost, and ease of use of the power supply. In portable systems the power supply of choice is typically some form of electrochemical battery. For the sake of economy, rechargeable battery packs are preferable to disposable batteries. A rechargeable battery pack has a much higher initial price, but their cost of use is significantly below that of disposable batteries.
The design of a rechargeable battery system depends on the expected use. A device in near constant use, such as a cellular phone, requires a battery pack which can provide the most operation time, and which can be recharged quickly. This has increasingly become the case since many portable devices are operated for extended periods of time. Accordingly, most rechargeable battery systems include a rapid charger which can fully recharge a battery pack in about an hour, although faster systems exist. The process of rapid recharging involves applying a high rate current to the battery pack and observing battery parameters such as voltage and temperature, and changes in those parameters, to determine when to cease rapid charging.
Of the existing rechargeable electrochemical systems, nickel cadmium (ni-cad) and nickel metal hydride (ni-meh) are most commonly used for portable electronic devices. Both of these systems generate heat when they are recharged in a rapid fashion. Ni-cad cells generally stay at a stable temperature until they are nearly fully charged. Ni-meh cells tend to produce heat throughout the charge cycle, but significantly increase heat output when they become fully charged. Coincidentally, the rate of change in temperature for both systems is nearly the same when they become fully charged.
Accordingly, many chargers have been designed to detect this characteristic while rapid charging a given battery pack so that the charger will stop charging before causing damage to the battery. The conventional approach to sensing and measuring the battery pack temperature has been to construct the battery with a temperature sensing element disposed internal to the pack and in close proximity to the battery cell or cells. This adds cost to the battery pack, but given the thickness and material typically used as casing in battery packs, it has been considered necessary for reliable temperature sensing.
However, in some applications the battery pack may not require a conventional housing. Indeed it would be a marketable advantage to use less expensive material for the battery pack case, and eliminate the temperature sensing element. The problem arises, then, as to how to accurately determine the temperature of a battery pack so constructed. Therefore, there exists a need for a battery charger that can determine the temperature of a battery pack constructed without a temperature sensing element, and reliably and safely recharge such a battery pack.